XML 141 R116.htm IDEA: XBRL DOCUMENT v2.4.1.9
Allowance for Losses (Details Textual) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Allowance for Losses (Textual) [Abstract]      
Allowances for trade receivables $ (5.7)us-gaap_LoansAndLeasesReceivableAllowance $ (5.2)us-gaap_LoansAndLeasesReceivableAllowance $ (4.6)us-gaap_LoansAndLeasesReceivableAllowance
Allowance for Trade Receivables [Member]      
Allowance for Losses (Textual) [Abstract]      
Allowances for trade receivables (4.5)us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_AllowanceForTradeReceivablesMember		 (4.0)us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_AllowanceForTradeReceivablesMember		  
Allowances for trade receivables as percentage of rent and other receivables 5.20%gmt_AllowancesForTradeReceivablesAsPercentageOfRentAndOtherReceivables
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_AllowanceForTradeReceivablesMember		 4.90%gmt_AllowancesForTradeReceivablesAsPercentageOfRentAndOtherReceivables
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_AllowanceForTradeReceivablesMember		  
Allowance for Finance Leases [Member]      
Allowance for Losses (Textual) [Abstract]      
Allowances for trade receivables $ (1.2)us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_AllowanceForLoanAndLeaseLossesMember